Modify logic for picking a default username value when lis_person_source...

...did parameter is not present.  It's believed that this should only be the case when user is added directly in Canvas UI and when admins switch to "Student View" mode.  Use custom_canvas_user_id (or user_id) with a class-level defined prefix in order to have a shorter field to get past a bug where the Test Student username was longer than 30 characters.